An optical communication system for a subscriber in which an optical fiber is laid directly to a home or thereby and by which various information can be provided attracts attention. In constructing such subscriber system, reduction in the cost of various optical components which are employed in a communication apparatus is an important issue. In particular, with regard to a light source module, an optical coupling between a semiconductor laser and a singlemode optical fiber is a key to the reduction in the cost thereof. Therefore, various techniques in which the coupling with a singlemode optical fiber is facilitated by integrating a spot size transformer with a light source are suggested. For example, H. Kobayashi et al.,"Tapered Thickness MQW Waveguide BH MQW Lasers", IEEE Photonics Technology letters, Vol. 6, No. 9, pp. 1080-1081 (1994) (hereinafter referred to as "first prior art") discloses a narrow radiation tapered waveguide integrated semiconductor laser.
However, in the first prior art, the tapered waveguide is placed between a p-InP layer and a InP substrate, and particularly the absorption loss at the tapered waveguide region reaches a level not negligible. Furthermore, while the width of the waveguide is constant, a significant difference in thickness between a laser region and tapered waveguide region is required. Therefore, the tolerance in the fabrication of the device and yield in production are not sufficient.
As another example, K. Kasaya et al., "Monolithically Integrated Laser with A Simple Laterally Tapered Waveguide", The Institute of Electonics Informations and Communications Engineers, digest of Autumn Conference 1993, p. 4-261 (1993) (hereinafter referred to as "second prior art") discloses an integration type device.
However, in the second prior art, the reproducibility in fabrication of the device is not sufficient since the width of a guide layer is processed to about 0.45 .mu.m by a standard etching after growing the planar guide layer etc. Further, to cover the guide layer with low concentration cladding layers, an undoped InP layer is to be grown only at a tapered waveguide region in the steps by the selective etching and selective re-growing. Therefore, the yield in production must be reduced since many growing steps are required.